from the looks of that panel it's a 9", but by the way that button hole isplaced, adding a second wouldn't look right or be in the right position for gameplay.
since you already have one made, then I guess you have your answer but I would have suggested just fillng that hole and drilling 2 new ones. not too hard and there are numerous ways to go about it.
check out my cocktail for a pic of the control panel, I think that's what you are going for. those are 9" panels. I had to drill an extra hole too, but they ended up looking okay. the right button is a new hole.
good luck!